Re: [PATCH v4 4/6] KVM: x86: Bump hypercall stat prior to fully completing hypercall

[Binbin Wu <binbin.wu@linux.intel.com>]

On 11/28/2024 8:43 AM, Sean Christopherson wrote:
> Increment the "hypercalls" stat for KVM hypercalls as soon as KVM knows
> it will skip the guest instruction, i.e. once KVM is committed to emulating
> the hypercall.  Waiting until completion adds no known value, and creates a
> discrepancy where the stat will be bumped if KVM exits to userspace as a
> result of trying to skip the instruction, but not if the hypercall itself
> exits.
>
> Handling the stat in common code will also avoid the need for another
> helper to dedup code when TDX comes along (TDX needs a separate completion
> path due to GPR usage differences).
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>

> ---
>   arch/x86/kvm/x86.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 13fe5d6eb8f3..11434752b467 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9979,7 +9979,6 @@ static int complete_hypercall_exit(struct kvm_vcpu *vcpu)
>   	if (!is_64_bit_hypercall(vcpu))
>   		ret = (u32)ret;
>   	kvm_rax_write(vcpu, ret);
> -	++vcpu->stat.hypercalls;
>   	return kvm_skip_emulated_instruction(vcpu);
>   }
>   
> @@ -9990,6 +9989,8 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
>   {
>   	unsigned long ret;
>   
> +	++vcpu->stat.hypercalls;
> +
>   	trace_kvm_hypercall(nr, a0, a1, a2, a3);
>   
>   	if (!op_64_bit) {
> @@ -10070,7 +10071,6 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
>   	}
>   
>   out:
> -	++vcpu->stat.hypercalls;
>   	return ret;
>   }
>   EXPORT_SYMBOL_GPL(__kvm_emulate_hypercall);